The present invention relates to a multi-layer reflection mirror for soft X-ray to vacuum ultraviolet ray used for incidence of light having a wavelength of less than 200 nm which is called soft X-ray to vacuum ultraviolet ray, at an incident angle closely perpendicular to a mirror surface.
In the past, there has been no reflection mirror having a high reflection factor (reflectivity) for a light having a shorter wavelength than an area called the vacuum ultraviolet which is directed perpendicularly or closely perpendicularly to the mirror plane, and a reflection factor has been less than 1% for the incident angle close to the perpendicular incident angle. In an oblique incident reflection mirror which has a relatively high reflection factor, it is necessary to adjust the incident angle between 1.degree. and 2.degree.-3.degree. relative to the mirror plane. Because the light has to be directed to the plane with a very small angle, a very large size mirror is required even for a fine light beam and the use thereof is difficult, and limited freedom of design of the optical system, and the fact that reflection mirror must be polished in order to have a high degree of planarity over a wide area makes the use thereof difficult.
As vacuum evaporation techniques have advanced in recent years, a multi-layer reflection mirror having a number of super-thin films laminated have been manufactured, and the reflection mirror having a high reflection factor by the use of interference has been put into practice.
In the area of X-ray and vacuum ultraviolet ray, refractive indices of most materials are represented by complex refraction coefficients (n+ik, hereinafter called refraction coefficients) having imaginary number portions k representing absorption, and real number portions n being substantially equal to 1.0 (n=1-.delta., .delta..perspectiveto.10.sup.-1 -10.sup.-3). Accordingly, a Flesnel reflection factor at a boundary of vacuum and material thin film is very small, that is, in the order of 0.1% or less. The reflection factor does not exceed several % per boundary plane ever at a boundary of laminated thin films of heterogeneous materials. By alternately laminating heterogeneous materials to form a multi-layer laminated structure so that reflected lights from the respective layer boundaries enhance each other by interference a reflection factor of the entire multi-layer film is maximized and, a high reflection factor is thus attained. By selecting a combination of heterogeneous materials which results in a big difference between refractive indices of adjacent layers to attain a high reflection factor together with the multi-layer film structure, a reflection mirror which has a high reflection factor at an incident angle close to a normal incident angle is attained.
In known combinations of materials, a transition metal element having a high melting point is used as a material for a low refractive index layer, and a semiconductor element such as carbon or silicon is used as a material for a high refractive index layer. Typical examples are combinations of tangsten (W) and carbon (C) and combinations of molybdenum (Mo) and silicon (Si). (See S. V. Gaponor et al, Optics Comm. 38 (1981), 7; T. W. Barbee et al, App. Opt. 24 (1985), 883).
When a high intensity light such as a synchrotron track radiation light is applied to a reflection mirror having such a combination, the reflection mirror is locally heated and the multi-layer structure will be readily broken if the high refractive index layer has a low melting point (for example, Si).
In order to avoid the above problem, the low refraction coefficient metal layer having a high melting point may be used, whereby but a metal single body usually has a melting point of around 2500.degree. C. (for example, Mo/Si) and a metal having a melting point of 3000.degree. C. or higher (for example, W/C) does not attain a high reflection factor.